What if the DGPS datum is different from that used for maritime charts?

DGPS is referenced to the World Geodetic System 1984 (WGS84) Datum. However at present no charts covering the waters within the GLAs' areas of responsibility produced by the Hydrographic Office of the United Kingdom are referred to WGS84.

Most DGPS receivers will have a facility to transform positions from WGS84 Datum to the regional datum of the chart, but the resulting accuracy will depend on the transformation parameters contained within the software of the user's receiver.

The GLAs strongly advise mariners to keep the DGPS receiver referenced to WGS84 Datum and to apply the position shift values denoted on the relevant nautical chart before plotting a position. Position shifts for those charts which do not include a "Satellite Derived Positions" note are listed in the UK Hydrographic Office Notice to Mariners 19/98. Reference should also be made to the current Volume 2 of the Admiralty List of Radio Signals.